As an apparatus for reducing the force required for operating the steering wheel when applying a steering angle to steered wheels of an automobile (normally the front wheels except in special vehicles such as a fork lift or the like), an electric power steering apparatus that uses an electric motor as an auxiliary power source are widely used. Although various constructions for an electric power steering apparatus are known, an auxiliary power from an electric motor is applied by way of a speed reducer to a rotating shaft that is rotated by the steering wheel operation in any of such constructions. Normally, a worm speed reducer is used as this kind of speed reducer. An electric power steering apparatus that uses a worm speed reducer can transmit an auxiliary power from an electric motor to a rotating shaft by engaging a worm that is rotated and driven by the electric motor with a worm wheel that rotates together with a rotating shaft.
FIGS. 9 and 10 illustrate an example of a conventional electric power steering apparatus. The front end portion of a steering shaft 2 that is rotated by a steering wheel 1 is freely rotatably supported on the inside of a housing 3, and a worm wheel 4 is fastened to a portion that is rotated and driven by the steering shaft 2. A warm 7 is connected to an output shaft 6 of an electric motor 5. An auxiliary torque having a specific size in a specific direction can be applied from the electric motor 5 to the warm wheel 4 by engaging the worm 7 with the worm wheel 4.
While operating this kind of electric power steering apparatus, direction and amount of electric current to the electric motor is controlled based on the direction and size of the torque that is applied to the steering shaft 2 from the steering wheel 1. A suitable auxiliary torque is then applied to the steering shaft 2 by way of the worm 7 and worm wheel 4. Thus, the torque that is transmitted to a steering gear unit 9 by way of an intermediate shaft 8 becomes larger than the torque that is inputted from the steering shaft 2. Accordingly, a pair of left and right tie rods 10 can be pushed or pulled by a larger force than the operation force that is applied to the steering wheel 1, so that a desired steering angle can be applied to a pair of left and right steered wheels with small operation force.
A worm wheel having composite construction combining a metal hub and synthetic resin gear portion as disclosed in JPH 11-301501 (A), JP 2001-206230 (A), JP2004-034941 (A), JP2004-345573 (A), and JP2006-022883 (A) is widely known as a worm wheel 4 that constitutes a speed reducer for an electric power steering apparatus. That is, the portion that is fitted around and fastened to the steering shaft is composed of a metal circular ring-shaped hub, and the portion that engages with a worm is composed of a synthetic gear portion. By making the gear portion using a synthetic resin, it becomes easier to form teeth around the outer circumferential surface of the worm wheel so that the manufacturing cost can be reduced. Moreover, gear rattle noise that occurs in the engagement portion between the worm wheel and the worm can also be reduced. FIG. 11 illustrates a worm wheel 4a having the construction that is disclosed in JP2004-034941 (A), and FIG. 12 illustrates a main part of an electric power steering apparatus in which the worm wheel 4a is assembled.
The worm wheel 4a comprises a hub 11 and a gear portion 12. The hub 11 is formed into a circular ring shape being made of metal such as a copper alloy or ferrous alloy. An circular installation hole 14 for fitting and fastening the hub 11 onto an output shaft 13 is provided in the center portion of the hub 11, an annular concave portion 15 that is recessed in the axial direction is provided in a portion that is near the inner diameter rather than the outer circumferential edge portion on one surface in the axial direction of the hub 11, and a spline portion 16 having concave and convex portions formed in the circumferential direction is provided on the outer circumferential surface of the outer circumferential edge portion of the hub 11.
In the meantime, a gear portion 12 is made of a synthetic resin such as polyamide resin, polyacetal resin in which reinforced fiber such as carbon fiber or glass fiber is mixed, and worm wheel teeth 17 are provided around the outer circumferential surface thereof. The gear portion 12 is formed by feeding a synthetic resin into a circular ring-shaped cavity that is formed between the portion near the outer diameter of the hub 11 and the inner surface of a mold with the hub 11 being set in the mold. When feeding, the synthetic resin is fed into the cavity from a ring gate that is located on the other surface in the axial direction of the hub 11 (right surface in FIGS. 11 and 12), and molded to cover the outer circumferential edge portion of the hub 11 around the entire circumference. Of the synthetic resin that forms the gear portion 12, the portion that corresponds to the inner circumferential edge portion of the gear portion 12 that is located on one surface side in the axial direction of the hub 11 fills the portion near the outer diameter of the annular concave portion 15 and forms a restraining portion 18. Further, the synthetic resin also fills concave portions of the spline portion 16, and connects the hub 11 and the gear portion 12 such that torque can be transmitted. Furthermore, when removing the synthetic resin from the portion that is connected to the ring gate, the portion near the outer diameter is left to become a second restraining portion 19. Because of this construction, the hub 11 and gear portion 12 are firmly connected. After taking out of the hub 11 and gear portion 12 from the mold, grinding as a finishing process is performed on the worm wheel teeth 17 of the outer circumferential surface of the gear portion 12.
As illustrated in FIG. 12, this kind of worm wheel 4a is fitted around and fastened to an output shaft 13 of an electric power steering apparatus. The output shaft 13 and the steering shaft 2 that is rotated and driven by the steering wheel 1 (see FIG. 9) are connected by a torsion bar 20. Moreover, a torque sensor 21 is provided on the inner surface of the housing 3 so as to be able to measure the torque that is transmitted between the steering shaft 2 and the output shaft 13. Based on the measured value of the torque, current flows to an electric motor 5, and an auxiliary torque is applied to the output shaft 13 by way of the worm wheel 4a. Because the construction and function of this kind of electric power steering apparatus as a whole are conventionally known widely, a detailed explanation thereof will be omitted.
An electric power steering apparatus is more used in comparatively large automobiles in recent years, and thus an auxiliary torque that is applied to the output shaft 13 by way of the worm wheel 4a tends to become large. When applying an auxiliary torque to the output shaft 13 by way of the worm wheel 4a, a moment M in the falling direction as illustrated by the arrow in FIG. 11 is applied to the worm wheel 4a from the gear engagement portion between the worm wheel 4a and the worm 7 (see FIG. 10). In the case of the worm wheel 4a illustrated in FIGS. 11 and 12, the strength of the connecting portion between the hub 11 and the gear portion 12 is not necessarily strong enough for this moment M. That is, durability may not be sufficiently maintained when this kind of construction is applied to an electric power steering apparatus that applies a large auxiliary torque.